U.S. Pat. No. 4,095,544 (NATIONAL STEEL CORPORATION) 20 Jun. 1978 details conventional Draw & Wall Ironing (DWI) and Draw & Re-Draw (DRD) processes for manufacturing can bodies from cup-sections for use in making two-piece metal containers. [Note that in the United States of America, DWI is instead commonly referred to as D&I]. The term “two-piece” refers to i) the can body and ii) the closure that would be subsequently fastened to the open end of the filled can body to form the container.
In a DWI (D&I) process (as illustrated in FIGS. 6 to 10 of U.S. Pat. No. 4,095,544), a flat (typically) circular blank stamped out from a roll of metal sheet is drawn through a drawing die, under the action of a punch, to form a shallow first stage cup. This initial drawing stage does not result in any intentional thinning of the blank. Thereafter, the cup, which is typically mounted on the end face of a close fitting punch or ram, is pushed through one or more annular wall-ironing dies for the purpose of effecting a reduction in thickness of the sidewall of the cup, thereby resulting in an elongation in the sidewall of the cup and forming a can body. By itself, the ironing process will not result in any change in the nominal diameter of the first stage cup.
FIG. 1 shows the distribution of metal in a container (or “can”) body resulting from a conventional DWI (D&I) process. FIG. 1 is illustrative only, and is not intended to be precisely to scale. Three regions are indicated in FIG. 1:                Region 1 represents the un-ironed material of the base 1. This remains approximately the same thickness as the ingoing gauge of the blank, i.e. it is not affected by the separate manufacturing operations of a conventional DWI process.        Region 2 represents the ironed mid-section 2 of the sidewall. Its thickness (and thereby the amount of ironing required) is determined by the performance required for the container body.        Region 3 represents the ironed top-section 3 of the sidewall. Typically in can making, this ironed top-section is around 50-75% of the thickness of the ingoing gauge.        
In a DRD process (as illustrated in FIGS. 1 to 5 of U.S. Pat. No. 4,095,544), the same drawing technique is used to form the first stage cup. However, rather than employing an ironing process, the first stage cup is then subjected to one or more re-drawing operations which act to progressively reduce the diameter of the cup and thereby elongate the sidewall of the cup. By themselves, most conventional re-drawing operations are not intended to result in any change in thickness of the cup material. However, taking the example of container bodies manufactured from a typical DRD process, in practice there is typically some thickening at the top of the finished container body (of the order of 10% or more). This thickening is a natural effect of the re-drawing process and is explained by the compressive effect on the material when re-drawing from a cup of large diameter to one of smaller diameter.
Note that there are alternative known DRD processes which achieve a thickness reduction in the sidewall of the cup through use of small or compound radii draw dies to thin the sidewall by stretching in the draw and re-draw stages.
Alternatively, a combination of ironing and re-drawing may be used on the first stage cup, which thereby reduces both the cup's diameter and sidewall thickness. For example, in the field of the manufacture of two-piece metal containers (cans), the container body is typically made by drawing a blank into a first stage cup and subjecting the cup to a number of re-drawing operations until arriving at a container body of the desired nominal diameter, then followed by ironing the sidewall to provide the desired sidewall thickness and height.
However, DWI (D&I) and DRD processes employed on a large commercial scale have a serious limitation in that they do not act to reduce the thickness (and therefore weight) of material in the base of the cup. In particular, drawing does not result in reduction in thickness of the object being drawn, and ironing only acts on the sidewall of the can body. Essentially, for known DWI (D&I) and DRD processes for the manufacture of can bodies for two-piece containers, the thickness of the base remains broadly unchanged from that of the ingoing gauge of the blank. This can result in the base being far thicker than is required for performance purposes.
The metal packaging industry is fiercely competitive, with weight reduction being a primary objective because it reduces transportation and raw material costs. Typically, containers such as cans for packaging food or beverage products are formed from a coil of single reduced steel of less than 0.35 mm thickness. By way of example, around 65% of the costs of manufacturing a typical two-piece metal food container with side walls ironed to 0.127 mm (0.005″ (5 thou)) derive from raw material costs.
There is therefore a need for improved light-weighting of metal cup-sections in a cost-effective manner.
Unpublished patent application PCT/EP11/051666 in the name of Crown Packaging Technology, Inc. describes a method of manufacture of a can body which uses a stretching operation to achieve a base which is thinner than the ingoing gauge of the metal sheet prior to stretching, without requiring loss or waste of metal. The present application is in the name of the same Applicant and represents an improvement of the invention of that unpublished application which relates to improving the effectiveness of the stretching process by completing it in two or more separate stretching stages. The improvement of the present invention increases stretch in previously unstretched and/or under-stretched portions of the cup-sections.
Note that in this document, the terms “cup-section” and “cup” are used interchangeably. Furthermore, the term “container” and “can” are often used to refer to the same product.